Such a tensioner is known e.g. from EP 0 657 662 A 2. This chain tensioner comprises a tensioning piston having a plurality of locking grooves on the outer circumference thereof, the locking grooves being engaged by a spring-loaded locking piston. The front face of the locking piston is beveled and can be acted upon by an oil pressure so as to disengage the locking position. The locking means guarantees that a locking effect is produced when the engine oil hydraulic system is switched off and that the locking piston can no longer retract. This has the effect that a certain pretension is maintained, even if the engine is at rest, in spite of possible oil leakage from the pressure chamber. Hence, a predetermined tension, which is independent of the engine oil hydraulic system, will exist when the engine is started. As soon as sufficient pressure has built up, the locking means will be disengaged and the tensioning piston will operate in the usual way.
A similar tensioner is known e.g. from DE 195 48 923 A1. In addition, conventional locking devices are provided on chain tensioners and the like, but these locking devices only serve to carry out a readjustment and to limit the retraction path of the tensioning piston to a pre-determined value. Such structural designs are used for wear compensation. Locking de-vices connected to the engine oil hydraulic system are disadvantageous insofar as they are subjected to the fluctuations of the hydraulic pressure and to a possible pressure build-up in the area of the tensioning piston. In the case of conventional locking devices, the locking piston is forced back by the teeth on the tensioning piston against the force of a spring, and this will entail wear.
It is therefore the object of the present invention to improve the structural design of a tensioner of the type used with chaims and belts.
In accordance with the present invention, this object is achieved by the features that the locking piston comprises an operating section causing said locking piston to move to the arresting position and to the release position, and a locking area used for engagement with the locking means of the tensioning piston and arranged separately from the operating section. Due to the fact that the operating point of the locking piston and the point of engagement are separated from one another, it is no longer necessary that the locking area is acted upon by operating forces so as to disengage the locking piston. In the case of hydraulically operated tensioners this means e.g. that the locking area of the locking piston need no longer be exposed directly to the engine oil hydraulic pressure. Actuation in one direction (arresting position) as well as in the other direction (release position) takes place at some other point. A great variety of operating mechanisms for achieving a piston movement can be employed. This separation of the locking function and of the operating function also permits the locking piston to be controlled in a purposeful manner in dependence upon the operating parameters. Conventional locking devices of the readjustment type as well as locking devices which are readjustable by an engine oil hydraulic system are always provided with operating means acting directly on the locking area.
According to an advantageous embodiment, the operating section of the locking piston can be provided with a piston area which is adapted to have applied thereto an operating pressure of a hydraulic fluid, the operating direction of the piston area being directed towards the release position of the locking piston. This measure has the effect that the locking piston will be displaced to the release position in response to application of a hydraulic pressure. Such a structural design could be used in internal combustion engines and connected to the engine oil hydraulic system.
In accordance with a preferred embodiment, a spring means can be provided, which acts on the operating area of the locking piston and which is effective in the direction of the arresting position of the locking piston. This measure has the effect that the locking piston is primarily forced into the arresting position by the spring force of the spring means. This means that an operating force in the opposite direction must always be larger so as to cause unlocking.
According to an advantageous embodiment, the locking area of the locking piston can be designed such that a hydraulic force balance is caused. Such a structural design allows the locking area of the locking piston to be subjected to a pressure medium, without the pressure medium causing any essential force component on the locking area for operating the locking piston. A person skilled in the art knows that, in order to achieve this, he must cause forces to act in opposite directions so that these forces will compensate each other (as far as the actuation is concerned). Hence, such a tensioner could definitely be arranged in the pressure chamber of the tensioning piston; pressure fluctuations of the pressure in the pressure chamber will not have any influence on the operation of the locking piston, neither into the arresting position nor into the release position. In hitherto used devices comprising controlled locking devices, such pressure fluctuations have always prevented the locking piston from moving immediately to its arresting position, when e.g. the engine hydraulic system had been switched off. The locking effect only occurred when the pressure had been reduced to a certain extent by leakage at the locking area of the locking piston. Such a delay is prevented by the structural design chosen.
In accordance with an advantageous embodiment, the locking area can be provided in the form of a locking opening in a locking plunger which extends away from the operating section, at least a portion of the locking means of the tensioning piston extending through the locking opening. The locking opening guarantees that a pressure medium in its interior will produce force components over the whole area of the opening and that the forces will compensate each other in such a way that the actuation of the locking piston will not be supported. This is a simple structural measure by means of which the locking area can be designated such that is not subjected to the influence of pressure forces.
In addition, the locking opening may be provided with a locking projection, which is arranged on the inner surface of the locking opening on one side thereof and which is used for engagement with the locking means of the tensioning piston. Such a locking projection on the inner side does not have any influence on the force conditions in the operating direction of the locking piston and guarantees nevertheless a reliable engagement with the locking means of the tensioning piston.
According to one variant, the locking means of the tensioning piston may comprise a lock-ing rod provided with teeth and extending through the locking opening of the locking piston. The operating paths of the tensioning piston and of the locking piston will therefore cross and individual components of the two pistons will interengage. A very compact and very robust structural design is provided in this way.
The locking rod can have a circular basic cross-section, the locking opening in the locking plunger being then implemented as an elongated hole which is adapted to this basic cross-section. This length (seen in the direction of the longitudinal axis of the locking piston) of the elongated hole can then correspond to at least to the operating stroke of the locking piston between the release position and the arresting position. This means that the locking rod and the locking piston part providing the locking opening can also mutually guide themselves, since the amount of play must be chosen precisely such that the locking rod can be dis-placed freely in the elongated hole in the release position. In the case of such a variant, the locking rod can also be implemented as an extension of the actual tensioning piston having a smaller diameter. However, also other cross-sections are possible instead of the circular cross-section.
When, in accordance with one variant, the inner surface of the locking opening is provided on one side thereof with an undercut portion which merges with the locking projection, the locking opening provides also in the arresting position a contact shoulder for close contact with the locking rod. When the locking piston is implemented as a plastic component or as a cast member, this undercut portion will also reduce the accumulation of material.
In accordance with one embodiment, the tensioning piston is guided in a housing, a pressure chamber is formed between the housing and the tensioning piston, the locking means extend from the inner to the outer side of the pressure chamber, and the teeth are located outside of the pressure chamber in the fully retracted position of the tensioning piston. This is to be regarded as an additional measure for displacing the locking area of the locking piston away from the pressure area of a fluid-operated tensioning piston. Depending on the structural design of the housing, the locking area will then only be subjected to a leakage flow of the fluid. Oscillating conditions occurring in the pressure medium during operation of the tensioning means will therefore not affect the locking piston.
According to a preferred embodiment, the locking piston can be guided in a housing such that it is separated from a pressure chamber of the tensioning piston. The two pistons can also be arranged in a common housing; in this case, only the locking means and the locking area cross each other and are adapted to be brought into engagement with one another.
According to a further embodiment of the tensioner for an internal combustion engine hav-ing an engine oil circuit, the piston area of the locking piston can be acted upon by the hy-draulic pressure of the engine oil circuit. By selecting an advantageous supply means, it can here be guaranteed that hydraulic oscillations of the type occurring in the area of the pres-sure chamber of the tensioning piston are decoupled as far as possible from the operation of the locking piston. In the simplest case, this delimitation is effected via a non-return valve.